1. Field of the Invention
This invention relates generally to an optical analog-to-digital converter and, more particularly, to an optical analog-to-digital converter that employs a downward-folding successive approximation conversion approach incorporating optical subtraction in successive stages, where saturable absorbers are used as the optical subtractors in one embodiment.
2. Discussion of the Related Art
Advances in signal processing technology, including the need for greater processing speeds, increased channel bandwidths and improved transmission reliability, has resulted in a steadily growing focus on the optical domain and the vast potential that lies therein with respect to these parameters. However, optical technology, as compared to electrical and radio frequency based technology, lacks the necessary technical sophistication in many areas. Particularly, the desire for high-speed, large-bandwidth processing devices employing digital optics has been hampered by the lack of many basic optical devices and technologies readily available in the RF domain.
Optical analog-to-digital (A/D) converters are one such device that has not heretofore met basic design requirements. Current digital optical systems rely on digital conversion in the electrical/RF domain. This requires conversions back and forth between the optical and RF domains that are slower, have more loss and are noisier than conversions in the optical domain only. An optical device that converts an optical analog signal to an optical digital signal with little or no reliance on RF technology would eliminate system complexity and provide for high speed and large bandwidth processing advances.
An especially difficult obstacle to optical analog-to-digital conversion is the need for optical subtraction. Adding optical signal intensity is relatively straight-forward and well understood, but the process of subtracting a specific amount of optical power from an optical signal is comparatively more difficult. U.S. patent application Ser. No. 09/133,138, filed Aug. 11, 1998, titled "Upward-Folding Successive-Approximation Optical Analog-To-Digital Converter and Method of Performing Conversion" and assigned to the assignee of this application, is directed to an optical A/D converter that provides an analog-to-digital conversion within the optical domain without the need to subtract optical signals. However, various applications may require an optical A/D converter that performs the conversion by employing optical subtraction.
What is needed is an optical A/D converter that employs subtraction of an optical intensity from an optical signal in an efficient and cost effective manner.